Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. Typically, a conventional magnetic element is used for storing data in such magnetic memories.
FIG. 1 depicts a conventional magnetic element 10, which may be a conventional magnetic tunneling junction (MTJ) or a conventional spin valve. The conventional magnetic element 10 may be used in a conventional magnetic memory. The conventional MTJ 10 typically resides on a substrate (not shown), uses seed layer(s) 11 and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional barrier layer 16, a conventional free layer 18, and a conventional capping layer 20. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. The ferromagnetic layers 14 and 18 typically include materials from the group of Ni, Co, and Fe and their alloys, such as CoFe, CoFeNi, which are low-moment ferromagnetic materials. For example materials such as FeCoB, FeCoTa, and FeCoBTa, with B from five through thirty atomic percent and Ta from five to twenty percent are used. Although depicted as simple (single) layers, the pinned layer 14 and free layer 18 may include multiple layers. For example, the pinned layer 14 and/or the free layer 18 may include two ferromagnetic layers antiferromagnetically coupled through a thin Ru layer via RKKY exchange interaction—forming a synthetic antiferromagnetic (SAF) layer. For example, a layer of CoFeB separated by a thin layer of Ru may be used for the conventional pinned layer 14 and/or the conventional free layer 18. The thin layer of Ru may, for example be between three and several tens Angstroms thick. The conventional free layer 18 is typically thinner than the conventional pinned layer 14, and has a changeable magnetization 19. The saturation magnetization of the conventional free layer 18 is typically adjusted between four hundred and one thousand four hundred emu/cm3 by varying the composition of elements. The magnetization 15 of the conventional pinned layer 14 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 12. Further, other versions of the conventional magnetic element 10 might include an additional pinned layer (not shown) separated from the free layer 18 by an additional nonmagnetic barrier or conductive layer (not shown).
Binary coded information, “1” or “0”, typically corresponds to the magnetization 19 of the free layer 18 and the magnetization 15 of the pinned layer 14 being parallel or anti-parallel, respectively. Thus, data are written by setting the magnetization direction(s) of the free layer(s) 18 in a specified cell. This is typically accomplished either by applying an external field, for example using a recording head (not shown) or using the spin transfer effect. To change the magnetization state of the free layer 18 using the spin transfer effect, a current is driven in a current-perpendicular to the plane (CPP) direction (i.e. the z direction in FIG. 1) through the conventional magnetic element 10 having a small enough size.
If a magnetic field is used to switch the magnetization state of the free layer 18, such a magnetic field is generated by current driven through conductive lines (not shown), such as a bit and digit lines (not shown). However, one of ordinary skill in the art will recognize that the currents required to write to the conventional magnetic element 10 do not scale down as the size of the magnetic memory cell using the conventional magnetic element 10 decreases. Because of this, a relatively high current may be needed to generate a magnetic field sufficient to switch the state of the conventional magnetic element 10. Consequently, power consumption of the MRAM using the conventional magnetic element 10 increases and the high more reliability issues may be encountered. Such a write scheme may thus be unsuitable for use at higher densities.
In contrast, the spin transfer effect is used to provide current based switching, the conventional magnetic element 10 may be written by driving a current through the conventional magnetic element 10. For spin transfer based switching to become important is switching the magnetization state of the conventional magnetic element 10, the lateral dimensions of the magnetic element 10 may be small, for example in the range of a few hundred nanometers or less, in order to facilitate current-based switching through the spin transfer effect.
In a conventional magnetic memory employing the conventional magnetic element 10 and utilizing spin transfer, the current required to switch the free layer 18 decreases as the magnetic memory density grows (which corresponds to a decrease in the lateral dimensions of the magnetic element 10). In particular, current may scale down in a manner comparable to the semiconductor or CMOS technology evolution. Consequently, the conventional magnetic element 10 has potential for use in higher density magnetic memories.
Although current density scales down with increasing densities for memories using the conventional magnetic element 10, one of ordinary skill in the art will recognize that there are still barriers to using the conventional magnetic element 10. In particular, a lower current is desired. The current density for spin-transfer switching to occur (switching current density) is in general greater than 107 A/cm2. For sub-micron lateral dimensions of magnetic cells, this high switching current density implies a high switching current (current at which spin transfer switching occurs) and thus a high bias current. For example, the bias current, Ic, may be greater than two mA for a conventional magnetic element having lateral dimensions on the order of ˜0.1×0.2 μm2).
This high bias current is undesirable for a number of reasons. A high bias current implies that the magnetic memory using the conventional magnetic element 10 has a higher power consumption during writing. Lower power consumption is generally desirable. In addition, each cell in a magnetic memory typically includes at least one conventional magnetic element 10 and at least one isolation transistor. The transistor size is proportional to the saturation current. In order to support a higher bias current, the isolation transistor would thus have larger dimensions. Larger dimensions of the isolation transistor would result in a larger memory cell size and a memory having lower density. Thus, a lower switching current corresponding to a lower bias current, a lower transistor size, and a higher density are desired. In addition, if a barrier layer is used for the layer 16 (making the conventional magnetic element 10 a conventional MTJ), a voltage across the layer 16 that is than dielectric breakdown voltage of the layer 16 is required to ensure that the conventional magnetic element 10 does not fail. Thus, a smaller current passing through magnetic element 10 cell for a given RA (resistance-area product) is desired. For the above reasons, a smaller switching current is desired for spin transfer based switching of the conventional magnetic element 10.
In order to determine the switching current density, modeling behind the spin transfer model may be used. According to a prevalent spin transfer model (“Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1-L5 (1996).), the switching current density can be expressed for the films having in-plane dominant anisotropy asJc∝αMstF(Heff+2πMs)/g  (1)where α is the phenomenological Gilbert damping constant, tF and Ms are the thickness and saturation magnetization of the free layer, respectively. The effective field Heff includes the in-plane uniaxial anisotropy field, the shape anisotropy field, the external magnetic field, and the dipolar and exchange coupling fields between the free layer 18 and the other multilayers in the magnetic element 10, g corresponds to an efficiency of spin transfer effect. In equation (1), the demagnetizing field 2πMs term dominates because 2πMs>>Heff for a typical ferromagnetic material (such as Co, Fe, Ni and their alloys). To reduce magnetization Ms of the free layer is an efficient way to reduce Jc, since Jc is proportional to Ms2. On the other hand, reducing the magnetization Ms results in the decrease of shape anisotropy and magnetic energy barrier which leads to the thermally-activated loss of the stored information. Consequently, without more, a reduction of the magnetization is not desirable for reducing the switching current of the conventional magnetic element 10.
Consequently, although the conventional magnetic element 10 has a possibility of being used in higher density memories, a lower switching current density is desired. Accordingly, what is needed is a method and system that may improve performance of the current-based switching of the conventional magnetic element 10. The method and system address such a need.